The rapidly increasing number of large, deep draft vessels, many of them carrying potentially dangerous and/or polluting-type cargos through poorly charted waters, has called attention to a severe lack of detailed precision hydrography in many areas of the world. The problem is especially acute in the coastal waters of some of the developing countries where new ports are opening almost daily in areas where detailed, reliable hydrographic data is either nonexistent or inadequate. With all the modern navigation methods available to today's seagoing skipper, such as SATNAV, precision radar, LORAN-C, Differential Omega and the like, there should be little doubt about the position of one's own vessel. However, accurate and detailed charts of the area are just as important to the completion of a safe passage as is accurate knowledge of a vessel's position. It is of little benefit to know precisely the position of one's own ship unless one also knows, with equal precision, the location of the channels, bars, shoals, and other navigational hazards in the areas. Hence, the rapidly developing demand for an enormous amount of detailed, accurate hydrography, which must be produced and published as rapidly as possible. At the same time, the rapidly escalating cost of hydrographic operations means that this expanding workload must be carried out using the most efficient and cost-effective methods available. This is, of course, the motivation behind the development of higher speed hydrographic vessels, more precise echo sounders, multiple transducer arrays and the experimental airborne hydrographic systems using infrared and laser systems.
One of the most important factors in the performance of accurate hydrographic surveying is the positional reference system by means of which the hydrographic data is referenced to the geodetic earth. In most cases today, the positional reference system will be one of several radiopositioning systems each of which has its own set of individual characteristics and capabilities. The criteria for an acceptable radiopositioning system include: a precision of better than .+-.5 meters; long range to cover coastal shelf areas: high reliability and freedom from down time; low cost; simplicity of operation; a minimum number of radio frequencies; the capability of supporting a large number of simultaneous users; and unambiguous identification and confirmation of position.